1. Field of the Invention
This invention relates to magnetic reproduction equipment including a video tape recorder (hereinafter referred to as a VCR) and particularly to a tracking control apparatus which automatically regulates relative-positional errors of a reproduction head and a track.
2. Description of the Prior Art
Image signals have so far been recorded in such a known VHS-system VCR records a video signal on a magnetic tape by running the magnetic tape at a constant speed to form helical tracks on it with a rotary head rotating synchronously with the vertical synchronizing signal contained in the video signal and, at the same time, records a control signal on the magnetic tape in a lengthwise direction with a control head installed at a specific distance from the rotary head. Then, at the time of video signal reproduction, the rotary head is rotated at a phase synchronized with a phase of a reference signal having the same frequency as that of the vertical synchronizing signal and the magnetic tape is run at the same speed as that at the time of recording. At that time, the positional coincidence between a helical track and the rotary head is achieved by making the phase of the control signal reproduced by the control head with respect to the phase of the reference signal (hereinafter referred to as the tracking phase) equivalent with the relationship between the phases of the vertical synchronizing signal and the control signal at the time of recording.
However, when the magnetic tape on which the video signal has already been recorded elongates or shrinks due to, for example, an increase in ambient temperature or there is any difference in mechanical accuracy of the tape running system between the VCR used for recording and the VTR used for reproducing the signal, the distance between the positions of the control signal and the helical track recorded on the magnetic tape no longer becomes the same as the distance between the positions of the rotary and control heads. This causes an error in the relative position of the rotary head to the helical track, which in turn causes a failure in reproduction of good-quality video signal. Such a tracking control device is known that is capable of automatically and gradually changing the tracking phase at, for example, the initial stage of signal reproduction to find out a point where the envelope level of the signal reproduced by the rotary head becomes maximum, and adjusting the tracking phase into this point, as disclosed in Japanese Patent Publication No. 63-104255 (1988).
However, due to the general occurrence of a change in the state of contact between the rotary head and the magnetic tape, the envelope level of the reproduced signal obtained at the rotary head fluctuates even if the tracking phase is constant. As a measure to remove such an effect from the conventional tracking control device, envelope level of the signal reproduced at a given tracking phase is sampled several times and averaged.
For further improvement of tracking accuracy, it is necessary to increase the number of the tracking phase changing steps. This, however, causes an increase of the time required to complete a series of tracking operation, because plural samplings, as mentioned above, of the reproduction envelope level are required in each step, or for each tracking phase.
Moreover, when the track width of the helical track differs from the head width of the rotary head, it is difficult for the conventional device to adjust tracking by simply determining the optimum point of tracking phase, because the reproduction envelope level response to the tracking phase takes the shape of upward convex trapezoid without having an exact peak.
Another conventional tracking control method has been disclosed in Japanese Patent Publication No. 2-276051 (1990). In this tracking control method, the phase relationship between the control signal recorded on the control track of a magnetic tape and rotation of the rotary drum provided with a rotary head (tracking phase) is gradually changed to detect a change of the signal level of the reproduced signal to obtain a curve showing the characteristic of the level of the reproduced signal changing with the change of the tracking phase. Then the barycenter of the characteristic curve is determined to control a capstan motor in accordance with the phase relationship in the barycenter obtained. In this method, however, tracking can not be controlled accurately when an accurate reproduced signal level change characteristic curve is not obtained due to noise and/or reproduced signal fluctuations. To reduce the effects due to noise and/or reproduced signals, it is necessary to repeat the measurement several times and obtain an averaged result, which requires more time to spend for tracking operation.